Nigerian security sources said between three and seven people were killed when militants ambushed the boat as it made its way from Cameroon.

Correspondents say security had been beefed up ahead of the ceremony.

On the Cameroonian side, there have been celebrations as people moved back into the peninsula.

In recent years, at least 100,000 people have moved from the peninsula to Nigeria, local leaders say.

The International Court of Justice ruling was based on an early 19th century colonial agreement between Britain and Germany.

Nigeria challenged the ruling, but finally agreed to relinquish the territory two years ago.

“The gains made in adhering to the rule of law may outweigh the painful losses of ancestral homes,” said the head of the Nigerian delegation at the ceremony, Attorney General Mike Aondoakaa.

Part of the territory was handed over to Cameroon two years ago.

Revellers

A spokesman for Nigerian President Umaru Yar’Adua said the process was “painful… for everyone including the president”, but added that Nigeria had made “a commitment to the international community and we have a responsibility to keep it”.